An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism, and performs assist control. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears (worm gears) 3 constituting the reduction mechanism, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, a torsion bar is inserted into the column shaft 2, for which a steering angle sensor 14 for detecting a steering angle θ of the steering wheel 1 by means of a twist angle of the torsion bar and a torque sensor 10 for detecting a steering torque Tt are provided, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control command based on the steering torque Tt detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value.
Moreover, the steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotational angle sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed V from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MPU, an MCU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tt detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 that calculates a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the inputted steering torque Tt and vehicle speed V and by using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current Im being fed back is calculated. The deviation I is inputted into a proportional integral (PI) control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37. The motor current Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 is comprised of a bridge circuit of field effect transistors (FETs) as semiconductor switching elements.
A rotational angle sensor 21 such as a resolver is connected to the motor 20, and a rotational angle θ is detected and outputted by the rotational angle sensor 21.
Further, a compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 34C and an inertia 34B at an adding section 34D, further adds the result of addition performed at the adding section 34D with a convergence 34A at an adding section 34E, and then outputs the result of addition performed at the adding section 34E as the compensation signal CM.
Research and development of automatic driving technique of a vehicle has been recently advanced, and proposals where an electric power steering apparatus (EPS) is applied to automatic steering included in the technique, have been made. In the case of achieving automatic steering by the EPS, the EPS has a mechanism for assist control performed by a conventional EPS and a mechanism for steering angle control of controlling a steering system so that a vehicle drives in a desired direction independently, and is generally configured so as to make outputs of these mechanisms possible to adjust. In the steering angle control, position and velocity control having superior performance of responsiveness to a steering angle command being a control target of a steering angle and a disturbance suppression characteristic to a road surface reaction force and so on, is used, for example, a proportional (P) control is adopted in position control, and a proportional integral (PI) control is adopted in velocity control.
In the case of performing the assist control and the steering angle control independently and performing the whole control by switching command values being outputs of both controls, switching them by a switch or the like suddenly may cause uncomfortable feeling to a driver because the command value is suddenly changed and behavior of a steering wheel becomes unnatural. In order to deal with this problem, in the case of switching between a torque control method (corresponding to the assist control) and a rotational angle control method (corresponding to the steering angle control), an apparatus disclosed in Japanese Unexamined Patent Publication No. 2004-17881 A (Patent Document 1) sets a value obtained by multiplying respective command values of both methods by coefficients (an automatic coefficient and a manual coefficient) and adding the multiplied results on a final command value, gradually changes these coefficients, and suppresses an abrupt change of the command value. The apparatus uses a P-control in position control of the rotational angle control method, and uses a PI-control in velocity control.
In the publication of Japanese Patent No. 3917008 B2 (Patent Document 2), an automatic steering control apparatus is proposed that automatically performs a steering wheel operation depending on a set steering angle and aims at parking assist in particular. This apparatus can switch between a torque control mode (corresponding to the assist control) and a parking assist mode (corresponding to the steering angle control), and performs the control by using prestored parking data in the parking assist mode. Further, the apparatus performs a P-control in position control of the parking assist mode, and performs a PI-control in velocity control.
The publication of Japanese Patent No. 3912279 B2 (Patent Document 3) does not directly apply the EPS, however, when an apparatus disclosed in Patent Document 3 starts steering angle control by switching a mode to an automatic steering mode, the apparatus reduces uncomfortable feeling to a driver caused by an abrupt change of a steering wheel at the start by gradually increasing a steering velocity (a steering angular velocity).